This R21 application is in response to the program announcement PA-03-058. The stated objective of this program announcement is to invite applications in exploratory or developmental bioengineering research (EBRG). One listed example of bioengineering research is: Research on drug, gene, or cellular therapeutic delivery systems. This proposal addresses this example. The ideal gene product delivery system would have four main characteristics. (1) It should be delivered with high efficiency; (2) it should exhibit prolonged gene expression (3) it should be show no toxicity; and (4) it should be applicable to the entire patient population without any need for customization. Working in a murine model, we propose to develop a gene delivery system that meets all these requirements. The proposed therapy involves the injection of an MHC-mismatched, transgene-expressing stromal cell line into the thymus. We will establish whether the recipient mice are tolerant to the injected cell line, despite the MHC mismatch, due to the unique nature of the thymic microenvironment (point 4 above). We will examine the recipients to confirm that the injected cells remained at the site of injection (point 1), and we will monitor long-term expression of the gene product (point 2) while looking for any possible pathology related to presence of the delivery cells in the thymus (point 3). Long term gene product delivery following intrathymic injection of transgene-expressing cells could revolutionize treatment of a number of single gene defect disorders. [unreadable] [unreadable] [unreadable] [unreadable]